The present invention relates to a circuit for cancelling a noise signal that occurs in an electrical signal while preserving the desired signal having the same frequency as the noise signal. The present invention is useful only in cancelling a noise signal which has a constant phase and amplitude. This type of noise signal occurs in many electrical signals as a result of the power distribution system of a country. In power distribution systems the earth is used as one conductor in the system, and considerable stray currents having a steady phase and amplitude occur. The presence of stray currents having the frequency of the power system is particularly troublesome in electrical circuits that are processing signals having the same frequency as the power supply. This occurs in seismic explorations where the frequencies of interest include the 60 Hz band which corresponds to the frequency of the power system in the United States. This can be appreciated by the fact that in a seismic survey the geophones are placed on the ground and coupled to a recording truck by means of cables which also lay on the ground. Thus, there exists a ready means by which the 60 cycle power signal may be superimposed on the geophone signals.
Several circuits have been suggested for solving the problem of 60-cycle power supply signals being superimposed on the seismic signals of interest. As explained in the above patent, some systems rely upon the use of notch filters which are adjusted to filter out the 60-cycle signals. While this is a satisfactory solution to the problem of the 60 Hz noise, it also removes 60 Hz geophone signals which are of interest. Also, various circuits have been designed for manually balancing out the 60 Hz noise.
The above patent specifically discloses and claims a system in which the noise signal is detected and used as a reference for generating a signal having precisely the same amplitude as the noise signal and precisely 180.degree. out of phase with the noise signal. While this system has been satisfactory and does cancel the noise signals, harmonics of the noise signal are not removed, and in effect, seriously effect the ability of the system to cancel the 60 Hz noise signal.